In recent years, demands for structural metal materials having high strength and light weight have increased in consideration of global environment. For example, in an automobile industry, there are growing demands for the safety of car body, and high-strength and light-weight parts of an automobile are increasingly required, so that parts of an automobile have been developed in order to improve fuel efficiency and collision safety.
In order to meet these demands, a high strength steel sheet having a much higher strength than that in the prior art, for example, a material having high strength with a tensile strength of 780 MPa or more, preferably, 900 MPa or more, has come into widespread use.
Meanwhile, while improving the strength of the steel sheet, the conventional structures of parts of an automobile have been reexamined. Following the above, there is a strong demand for the development of an art for accurately bending a metal material in any of various shapes, such as an art for two-dimensionally or three-dimensionally bending a metal material in different directions, in order to apply to various types of parts of an automobile.
In order to meet the demands for the development of the bending technology, various processing techniques have been proposed. For example, Japanese Patent Application Publication No. 50-59263 and Japanese Patent No. 2816000 disclose a method for bending a metal tube or the like while performing a thermal treatment on the metal tube or the like. Specifically, the following methods are disclosed: a bending method for clamping a leading end of a metal tube or the like with a rotatable arm, heating the metal tube or the like by using a heating unit, appropriately moving the heated portion of the metal tube or the like to bend the heated portion, and cooling down the bent portion (Japanese Patent Application Publication No. 50-59263); and a method for applying torsional and bending force to the heated portion of the metal tube or the like to bend the metal tube or the like while twisting the metal tube or the like (Japanese Patent No. 2816000).
However, the disclosed bending methods are so-called grab bending methods requiring a rotatable arm for clamping the leading end of a metal tube or the like, which makes it difficult to feed the metal tube or the like to be bent at high speed. In addition, the arm needs to make a return movement in order to repeatedly clamp the metal tube or the like, resulting in a remarkable variation in the feeding speed of the metal tube or the like. Therefore, a complicated control is required for a heating or cooling speed, which makes it difficult to ensure predetermined quenching accuracy.
In order to solve the above-mentioned problems of the grab bending method, Japanese Patent Application Publication No. 2000-158048 discloses a high-frequency heating bender based on push bending that supports a push bending roller so as to be movable in three dimensional directions. According to the high-frequency heating bender disclosed in Japanese Patent Application Publication No. 2000-158048, the push bending roller is swung around a workpiece to be bent toward the opposite side of the workpiece, and comes into contact with that, thereby bending the workpiece. Therefore, in a two-dimensional continuous bending operation in which a workpiece is two-dimensionally bent in different directions in, for example, an S shape, a procedure of turning the workpiece by 180 degrees is not needed.
However, in the high-frequency heating bender disclosed in Japanese Patent Application Publication No. 2000-158048, since there is no resort to clamp both side-faces of a workpiece to be bent, the workpiece is likely to be deviated from the intended shape due to the residual stress caused by a cooling operation after the high-frequency heating. Therefore, it is difficult to ensure predetermined dimensional accuracy, which makes it difficult to improve the accuracy of bending, while restricting the processing speed of the workpiece.